In Long Term Evolution (LTE) and successor systems (for example, also referred to as LTE Advanced (LTE-A), 4G, Future Radio Access (FRA), 5G, and the like) of LTE, Device to Device (D2D) technology has been studied in which units of user equipment directly communicate with each other without going through a radio base station (for example, Non-Patent Document 1).
D2D reduces the traffic between the user equipment and the base station, or enables communication between units of user equipment even when communication with the base station is not possible in the event of a disaster or the like.
D2D is classified roughly according to D2D discovery (D2D discovery, which is also referred to as D2D detection) for finding any other communicable user equipment and D2D communication (also referred to as D2D direct communication, D2D communication, inter-terminal direct communication, and the like) for performing direct communication between units of user equipment. Hereinafter, when D2D communication, D2D discovery, and the like are not particularly distinguished from each other, these are simply referred to as D2D. In addition, a signal transmitted and received by D2D is referred to as a D2D signal.
In addition, in 3rd Generation Partnership Project (3GPP), it has been studied to implement V2X by extending the D2D function. Here, the V2X is a part of Intelligent Transport Systems (ITS). As shown in FIG. 1, the V2X is a generic term for Vehicle to Vehicle (V2V) that means a mode of: communication performed between vehicles, Vehicle to Infrastructure (V2I) that means a mode of communication performed between a vehicle and a Road-Side Unit (HSU) installed at the side of the road, Vehicle to Nomadic device (V2N) that means a mode of communication performed between a vehicle and a mobile terminal of the driver, and Vehicle to Pedestrian (V2P) that means a mode of communication performed between a vehicle and a mobile terminal of a pedestrian.